Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a spinal cord stimulation system, such as that disclosed in U.S. Pat. No. 6,516,227 (“the '227 patent”), issued Feb. 4, 2003 in the name of inventors Paul Meadows et al., which is incorporated herein by reference in its entirety.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. As shown in FIGS. 1 and 2, a Spinal Cord Stimulation (SCS) system typically includes an Implantable Pulse Generator (IPG) or Radio-Frequency (RF) transmitter and receiver 100 (collectively, “IPGs”), at least one electrode lead 102 and/or 104 having a plurality of electrodes 106, and, optionally, at least one electrode lead extension 120. The electrodes 106 are arranged in a desired pattern and spacing on the lead(s) 102, 104 to create an electrode array 110. Wires 112, 114 within one or more leads(s) 102, 104 connect each electrode 106 in the array 110 with appropriate current source/sink circuitry in the IPG 100.
In an SCS application, the electrodes lead(s) 102, 104 with the electrodes 106 are typically implanted along the spinal cord 19 (FIG. 2B), and the IPG 100 generates electrical pulses that are delivered through the electrodes 106 to the nerve fibers within the spinal column. The IPG 100 body itself is normally implanted in a subcutaneous pocket, for example, in the patient's buttocks or abdomen. The electrode lead(s) 102, 104 exit the spinal column and generally attach to one or more electrode lead extensions 120 (FIG. 2), which in turn are typically tunneled around the torso of the patient to the subcutaneous pocket where the IPG 100 is implanted. Alternatively, if the distance between the lead(s) 102, 104 and the IPG 100 is short, the electrode lead(s) 102, 104 may directly connect with the IPG 100 without lead extensions 120. For examples of other SCS systems and other stimulation system, see U.S. Pat. Nos. 3,646,940 and 3,822,708, which are hereby incorporated by reference in their entireties. Of course, an IPG 100 is an active device requiring energy for operation, which may be provided by an implanted battery or an external power source.
Precise placement of the lead(s) 102, 104 relative to the target nerves is important for achieving a satisfactory physiological response, and for keeping stimulation thresholds low to conserve battery power. A conventional lead implantation procedure commonly places the leads 102, 104 parallel to the spinal cord column 19 at or near the physiological midline 91, as is shown in respective perspective and cross-sectional views in FIGS. 3A and 3B. More particularly, and as best shown in FIG. 3B, the electrode leads 102, 104 are placed directly on the dura mater 51 within the epidural space 70. (Cerebro-spinal fluid 72 is between the electrode array 110 and the white matter 52 of the spinal cord 19. Dorsal root nerves 50 are shown emanating from grey matter 53). When the leads 102, 104 are placed on opposite sides of the physiological midline 91 as shown, additional flexibility is provided in the ability to recruit (i.e., stimulate) nerves in the dorsal column, and to treat symptoms manifesting on either the left or right sides of the patient's body.
In addition to precise placement of the electrode array, proper selection of the electrodes, i.e., determining which of the electrodes 106 in the array should be active in a given patient, is critical for achieving effective stimulation therapy. However, because of the uncertainties of the distances of the electrodes from the neural target, the unknown nature of the specific conductive environment in which the electrode is placed, etc., it generally cannot be known in advance and with precision which combination of active electrodes will be perceived by a patient as providing optimal therapy. As a result, patient therapy generally requires at the outset that various electrode combinations be tried and feedback received from the patient as to which of the combinations feels most effective from a qualitative standpoint, what is referred to herein as IPG “set up.”
Various electrode combinations and other stimulation parameters can be tried during set up by programming the IPG 100, for example using the clinician programmer 204 or a hand-held programmer 202 (see FIG. 7, discussed below). For example, and as best visualized in FIG. 3A, the IPG 100 can be programmed such that electrode E1 comprises an anode (source of current), while E2 comprises a cathode (sink of current). Or, the IPG 100 can be programmed such that electrode E1 comprises an anode, while E9 comprises a cathode. Alternatively, more than one electrode can be used in both the sourcing and sinking of current. For example, electrode E1 could comprise an anode, while both E2 and E9 can comprise cathodes. Of course, the amount of current sourced or sunk can also be programmed by the IPG 100. Thus, in the last example, electrode E1 could sink 5 mA, while electrode E2 sources 4 mA and electrode E9 sources 1 mA. The frequency of electrode stimulation pulses, as well as the pulsewidth of such stimulation pulses, is also programmable.
Ultimately, which electrodes are activated by the IPG 100, and the polarities (cathode v. anode) and magnitudes (amount of current) of those activated electrodes, are based largely on patient feedback during IPG set up as noted earlier. Thus, the patient, usually with the benefit of a clinician, will experiment with the various electrode settings, and will report relative levels of comfort and therapeutic effectiveness to arrive at electrode settings that are best for a given patient's therapy.
Generally, and as one skilled in the art will appreciate, cathodic stimulation across the dorsal column (e.g., across the physiological midline 91) is preferable to cathodic stimulation across the dorsal roots 50. What this means in FIG. 3A is that cathodic stimulation from left to right (which promotes recruitment of the dorsal column) is generally preferable to cathodic stimulation from top to bottom (which promotes recruitment of the dorsal roots 50). In other words, generally, it is preferable to activate, for example, electrodes E1 and E9 (left to right, or from lead 102 to lead 104) as cathodic sinks as compared to electrode E1 and E2 (top to bottom, or along either lead 102 or 104 individually). This being said, this is merely a preference and not an inviolable rule, as ultimately which contacts are activated is a matter of patient's subjective preference.
In the prior art, patients and/or clinicians used a technique called “field steering” or “current steering” to try and simplify the iterative process for determining a patient's optimal electrode settings during set up of the IPG. See U.S. Pat. No. 6,909,917, which is incorporated herein by reference in its entirety. In current steering, the current sourced or sunk by the electrodes is gradually redistributed by the patient or clinician to different electrodes using a single stimulation timing channel. Such steering can be facilitated using some sort of user interface associated with an external programmer 202 or 204, such as a joystick or other directional device 206 (see FIG. 7). Examples of current steering are shown in FIGS. 4 and 5. Starting first with FIG. 4, assume that the IPG 100 has certain initial conditions, namely that electrode E1 has been programmed to source 10 mA of current, while electrode E9 has been programmed to sink 10 mA of current. This initial condition might be arrived at after some degree of experimentation, and might be a condition at which the patient is feeling a relatively good response, but a response which has not yet been fully optimized.
In an attempt at further optimization, current steering can commence from these initial conditions. Thus, in FIG. 4, suppose electrode E1 is selected and the current sourced from that electrode is to be moved downward (e.g., by clicking downward on the joystick). As shown, this moves 2 mA of sourcing current from electrode E1 (8 mA) to electrode E2 (2 mA). Another downward click moves another 2 mA, so that now E1 sources 6 mA and E2 sources 4 mA. Selection of sink electrode E9, followed by yet another downward click moves 2 mA of sink current to electrode E10 as shown. Current steering may also occur from left to right, i.e., from between leads 102 and 104. For example, it can be seen in the last step of FIG. 5 that 2 mA of source current has been steered from electrode E2 to electrode E10.
Gradual steering of the current in this manner (e.g., in increments) is generally considered advisable to safeguard against abrupt changes of the stimulation field which may be uncomfortable or dangerous for the patient. For example, assume from the initial condition in FIG. 4 that the patient feels relatively good coverage. If this is the case, it might be useful to try moving the cathode around, from E9 to either E2 or E10 for example, to see if even better coverage could be afforded the patient. However, it would generally be unadvisable to abruptly put the entirety of electrode E9's sink current (−10 mA) onto electrodes E2 or E10. Even though these electrodes are physically close to electrode E9, to place the full sink current onto these electrodes could have unforeseen and undesirable effects. Different nerves would certainly be affected by such a change in electrode activation, and it is not necessarily known how moving the full sink current would affect those nerves. If the current when applied to the new electrodes (e.g., E2 or E10) is too low (i.e., sub-threshold), no clinical response would be noticed, even if the electrodes were ultimately suitable choices. If the current is too high (i.e., supra-threshold), the result might be painful (or dangerous) for the patient. Accordingly, incremental movement of the current was considered the best approach.
However, such current steering, particularly in increments, has drawbacks. For example, consider the hypothetical shown in FIG. 6. Suppose initially that the patient perceives good coverage from the initial condition depicted in FIG. 6A, in which the active electrodes are tightly clustered along one lead 102. As shown, electrodes E1 and E3 each provide a 5 mA source current, while the middle electrode, E2 sinks the sum of that current, 10 mA. These initial conditions may suggest that a relatively similar combination of electrodes, but shifted by one electrode (E2-E4), would be reasonable to try as a target final condition, as shown in FIG. 6B. Not only may such shifting of electrodes be advisable during set up of the IPG 100, such adjustment may be necessary in an once-previously-optimized system should the lead 102 or 104 longitudinally slip along the spinal column 19 due to patient physical activity.
In any event, for whatever reason, it may be reasonable to simply try applying the conditions on electrodes E1-E3 on electrodes E2-E4. Using the current steering technique of the prior art, and recognizing the advisability of incremental steering of current between electrodes, the result of moving the conditions of electrodes E1-E3 to electrodes E2-E4 is slow and subject to erroneous results. Thus, as is illustrated in the sequential steps of FIG. 6C, the settings for the electrodes had to be incrementally “inch-wormed” into their new positions. Thus, the conditions at electrode E3 are first moved to E4 over a series of incremental steps. This is necessary to free electrode E3 to receive new settings, because E3 can't simultaneously respond to its old and new settings, i.e., electrode E3 cannot simultaneously source and sink anodic and cathodic current, respectively. Then, once E3 is free, E2's conditions are incrementally moved to E3. Then, once E2 is free, E1 is moved to E2 in like fashion. Thus, many steering steps are required to fully move the initial conditions on electrode E1-E3 to electrodes E2-E4. If nothing else, this is time consuming and cumbersome.
More importantly, this method of steering the current during set up in the hypothetical example of FIG. 6C can be subject to erroneous results. Suppose that the initial conditions (FIG. 6A) are a reasonable starting point for a particular patient, but that the target final conditions (FIG. 6B) would be even better for the patient. Because the prior art steering technique requires many intermediary steps between the initial conditions and the desired final conditions, it is possible that these intermediary steps could inadvertently dissuade the patient from discovering the benefits of the target final conditions. For example, notice that in the intermediary steps, all four electrodes E1-E4 are utilized to varying degrees. These intermediary steps do not necessarily bear a good relation to either the initial conditions (generally good) or the final conditions (even better). For example, in intermediary step 111a, electrode E3 draws no current at all, although in the final condition E3 should be drawing all of the sink current (10 mA). It is therefore not surprising that intermediary step 111a might not feel optimal for the patient. Specifically, the patient may find the intermediary steps uncomfortable, or the patient may not feel any stimulation effect or therapeutic relief whatsoever. In short, there is a risk that if the intermediary conditions are not perceived by the patient or clinician during set up as steps taken in the “right direction” towards more effective electrode settings, the plan to move the settings to the final conditions may be abandoned, even though with patience it would have been advisable to continue implementing this plan.
Moreover, because in the particular example of FIG. 6C the cathodic shifting occurs up and down along the lead, the negative effect of non-optimal intermediary conditions is potentially exacerbated. This is because movement of the cathode up and down a particular lead will tend to recruit different dorsal roots 50. As noted above, it is generally not preferred to stimulate the spinal column in this manner.
Accordingly, what is needed is an improved method for optimizing electrode activation during the set up of an implantable stimulator device, and this disclosure provides embodiments of such a solution.